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Linux is built for networking. The code associated with many standard networking services is integrated into the Linux kernel. A basic understanding of the functionality of standard Linux networking services is essential. Many themes throughout this book assume that you already understand the purpose of network communication protocols, mail services, host name and IP address management, Web services, and more.
In Red Hat Enterprise Linux, network services are often installed separately. Some include different packages for clients and servers. Some network services are activated through /etc/xinetd.conf, which reads activation files in the /etc/xinetd.d directory. Others are activated directly with scripts in the /etc/rc.d/init.d directory. I briefly examine some key RHEL network services in the following sections.
The first network system on Unix and Linux computers is the Network File System (NFS). Ideally, this leads to a seamless Linux interface; for example, you can set up one /home directory for all users on your network on one server. Remember, you need NFS on both server and client computers on your network.
First, make sure NFS support is part of the kernel, as documented in /proc/filesystems. If it isn't there, you may need to activate the nfs and related modules (nfsd, lockd, sunprc) in the kernel. Inspect installed modules with the following command:
# lsmod | more
Make a list of the modules that aren't included in the list. Run a modprobe command (for example, modprobe nfs) on any missing modules. With a standard Red Hat Enterprise Linux installation, this should add the modules to the lsmod list, and then add them to the kernel, as listed in /proc/filesystems.
Once you've shared an NFS directory, you can then activate the NFS daemon with the service nfs start command. I illustrate an example where you can share the Red Hat Enterprise Linux installation files through NFS in Chapter 2.
Once NFS is configured, you can find shared directories on the server's /etc/exports file, and then mount them with a command similar to the following:
# mount -t nfs nfsserver:/home /mhome
For more information on NFS, see Chapter 9.
There are some who suggest that sendmail is the biggest test for Linux system administrators. The sendmail configuration files, sendmail.cf and submit.cf, are complex. But it should not be intimidating. With the help of the corresponding .mc files, it's easier to define the features you want, the protocols you need, and the way mail is sent and received on your network.
More information on sendmail is available in Chapter 7.
The Post Office Protocol (POP) and the Internet Mail Access Protocol (IMAP) are each a set of rules for delivering e-mail from a server such as sendmail to an e-mail client such as Netscape, elm, or pine. While POP3 is the current standard for e-mail that is sent to clients, IMAP4 is more flexible for users such as those who access their mail using different computers. POP3 and IMAP4 configuration is addressed in Chapter 7.
Perhaps the most basic file sharing protocol still in common use is the File Transfer Protocol (FTP). It is set up specifically for file transfers; you might already know that file transfers using FTP are generally faster than with any other protocol.
As with NFS and Samba, you need a server and a client. FTP servers can be anonymous, which means they accept connections from anyone, or they can be configured to require a specific username and password. Generally, Linux FTP servers share files from the /var/ftp directory. Red Hat Enterprise Linux now comes with the Very Secure FTP daemon (vsFTPd) as the only FTP server.
The original FTP client works from the command line. Most Linux navigational commands work for FTP; just remember that the get and put commands download and upload specific files. FTP is covered in more detail in Chapter 7.
If there were a practical way to list all of the domain names and IP addresses of every Web site on the Internet in a single file, we would not need the Domain Name Service (DNS). The DNS system allows us to set up different parts of this database on different servers around the world. If a DNS server does not have the answer, you can configure it to ask other DNS servers for help. DNS is covered in more detail in Chapter 9.
IP version 4 addresses are scarce. The Dynamic Host Configuration Protocol (DHCP) was designed to help ration IP addresses. A DHCP server leases a specific IP address to a computer network card for a limited, but renewable, amount of time. DHCP servers can lease IP addresses on different LANs using the BOOTP protocol. More information on setting up DHCP clients and servers is available in Chapter 9.
The network system originally developed for networks with Microsoft and IBM computers is based on the Server Message Block (SMB) format. SMB, also known as Samba, is the basis for Microsoft Windows Workgroup and Domain-based network communication. When you install Samba on a Linux computer, you can make it part of one of these Microsoft-style networks. It can share files just like any other member of a workgroup network. It can act as a server. Current versions of Samba can even be configured as a Windows NT-style Primary Domain Controller or a member server on a Windows 2000/XP-based network.
Separate packages are available to set up your Linux computer as a Samba client and as a Samba server. Once shares are configured in /etc/samba/smb.conf, other Samba-enabled Linux clients can mount these directories with a command similar to the following:
# mount -t smbfs -o username=user //servername/sharename /mountpoint
You can also set up the smbmount command for this purpose. Samba and the associated configuration tools are discussed extensively in Chapter 8.
Apache is by far the most popular Web server in use on the Internet. It's a standard part of the Red Hat Enterprise Linux server installation. The main configuration file is /etc/httpd/conf/httpd.conf. Configuration is based on an extensive array of modules in the /etc/httpd directory. Basic HTML files, icons, and CGI applets are installed in the /var/www directory. The main Apache log files are part of the /var/logs/httpd directory. Daily log files for the largest Web sites can grow into the GB range. Apache is covered in more detail in Chapter 7.
A substantial number of other Web servers are available for Red Hat Enterprise Linux, such as Sun's iPlanet and Zeus's Web server.
The Network Information Service was formerly known as the 'yellow pages,' as it is a centralized database of usernames and passwords on a network with Linux and other Unix-style computers. NIS can be configured as a centralized database for a number of other configuration files in the /etc directory. Anything that can standardize the configuration of different computers on a network helps the system administrator. For more information on NIS, see Chapter 10.
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